The significance of partial suppressibility of serum thyroxine by triidothyronine administration in euthyroid man.

Recent evidence indicates that triiodothyronine (T3) administration may not completely inhibit normal thyroid secretion. To further corroborate this observation, measurement of serum T4-RIA concentrations was performed on 15 normal controls (10 men, 5 women; ages 20-42) who were placed on 100 mug of T3 daily for a 5-week period. Decrements of 53%, 36%, and 28% from the baseline T4-RIA were noted at weeks 1, 2, and 3 respectively. At 3 weeks a nadir T4-RIA of 2.5 mug/100 ml was reached which did not significantly differ from the 4th (2.9 mug/100 ml) and 5th weeks (2.6 mug/100 ml). Further, seven euthyroid patients who had received replacement thyroid hormone for 1-16 were switched to T3 (75-100 mug/day) for 28 days. At the end of this period, their mean T4-RIA was 2.6 mug/100 ml. Similar T3 treatment studies were performed on 20 primary hypothyroid patients. After 4 weeks of T3 all 20 patients displayed a T4-RIA below the limits of assay detectability (less than 0.625 mug/100 ml) while all euthyroid subjects had values greater than 1.2 mug/100 ml. Suppression of T4-RIA with T3 was also noted in 4 patients with pituitary and 2 patients with hypothalamic hypothyroidism. Three days after cessation of T3 treatment in normal subjects, no significant rise in mean T4-RIA was seen (2.3 mug/100 ml). Subsequently, T4-RIA rose to 4.5 mug/100 ml on day 7 and 6.7 mug/100 ml on day 10 (74% of the presuppression value) in normals. A similar rise to 7.9 mug/100 ml 10 days after withdrawal from T3 was noted in the euthyroid subjects who had received long-term thyroid hormone replacement. In contrast, all primary hypothyroid patients had either a minimal or nondetectable elevation in T4-RIA while demonstrating a marked rise in TSH 10 days after T3 withdrawal. An absent or impaired rise in T4-RIA after T3 withdrawal was also noted in patients with pituitary and hypothalamic hypothyroidism. These observations indicated: 1) There is continued thyroidal T4 secretion in euthyroid subjects receiving 100 mug of T3 daily. 2) The hypothesis is advanced that an intact hypothalamic-pituitary-tyhroid axis may be required for continued T4 secretion while on T3. 3) The duration of prior suppression with thyroid hormone medication does not appear to influence this phenomenon.